Dock levelers are used to transfer goods between a building and a truck trailer. Dock levelers bridge the gap between the building floor to the bed of the trailer or similar carrier. Dock levelers include a frame or support structure for mounting the leveler in a pit of a loading dock. The rear end of a conventional dock leveler is hinged to the building floor. The opposite end has an extendable lip plate that pivots out and onto the trailer bed. Levelers are adapted to move from a generally horizontal position where the upper surface of the deck is flush with the surface of the building floor to a second generally inclined position to provide a ramp between the bed of the truck and the dock floor.
Various mechanisms have been developed for extending and retracting the lip of the deck of a loading dock leveler during operation. The lip pivots from a hanging or pendant position to an extended position when the ramp is rising or when the operator walks the ramp down. The lip is typically extended by an actuator and held in its extended position by another mechanism. Examples of such lip extension mechanisms are disclosed in U.S. Pat. Nos. 3,137,017 to Pfleger, 3,997,932 to Artzberger, 4,398,315 to Driear, 5,475,888 to Massey, and 6,834,409 and 7,013,519 to Gleason, the disclosures of which are incorporated by reference herein.
A problem with conventional dock levelers is that the wider, longer or thicker the lip, the harder it is to extend and hold the lip in position. A loaded spring is typically used to assist in extending the lip. Although the assist spring is loaded at all times, the available force of the assist spring is contained when the lip is in its pendant or parked position. The assist spring typically does not start to work until the dock leveler is raised and the lip has already begun to extend. This loss of effective power occurs at the start of its extension, when the assist spring is needed most. Yet, increasing the tension or force of the assist springs makes it harder to walk the unit down.
Another problem with conventional dock levelers is that they tend to lose their ability to fully extend their lip. The lip extension mechanism begins to fail due to wear and other environmental considerations, a lack of lubrication or spring fatigue. Instead of latching and holding in an outward position, the lip tends to fall back to its pendant position. The leveler remains inoperative until proper lip extension is restored by periodic preventive maintenance or adjustment of the springs. Even a short outage can be significant given that these devices typically operate in a heavy industrial context.
Other levelers replace the mechanical latch with a hydraulic damper that permits the lip to be extended freely but restrict its retraction. Even if the lip is not fully extended, the damper will retard its retraction long enough for the operator to walk the leveler down to the truck bed. Hydraulic dampers also quickly retract under high load, which can protect the lip mechanism from damage when the lip is accidentally struck by a truck that backs into the lip while still extended. Still, hydraulic dampers have two significant drawbacks. First, the damper begins to retract as soon as the load is applied, and the operator must walk the leveler down immediately. Second, the viscosity of the hydraulic fluid is sensitive to changes in temperature. In warm temperatures, the lip falls too quickly for it to come to rest on the truck bed. In cold temperatures, the lip falls too slowly when the leveler is removed from the truck.
A further problem with conventional dock levelers is that the lip extension mechanism is complex and includes many components. One mechanism extends the lip, a different mechanism holds the lip extended, and a still further mechanism releases the lip so it can return to its stored position. These mechanisms and their various components increase the cost to manufacture, assemble and maintain the machine. These various mechanisms and components also tend to decrease the range of motion of the leveler. The leveler cannot be raised or lowered beyond certain upper and lower limits because of the tight or cramped arrangement of the many moving parts.
The present invention is intended to solve these and other problems.